The big box stores sell at least two grades of concrete. One grade has coarse stone, and not much sand and cement. It is not good for much except setting posts.
I used a good bit in the past week that has more cement, is rated at 4000 psi or 5000 psi, and is specified for parts as little as 2" thick. That is what you should select for sidewalks.
You need forms, probably a 2x4 on each side held in place with good stakes. They should be carefully adjusted with a level and/or a chalk line so the top of the form is the elevation of the finished sidewalk and is even without humps and valleys.
Then you need a screed, which is a 2x4 on edge, long enough to let you work it back and forth on top of the forms. It is nice if you find a crooked 2x4 and use it with the arc up so there is a little crown in the sidewalk so it drains.
After you pour excess concrete in the form starting at one end, you spread it with the screed. You want to pound on the forms with a hammer to get rid of the honeycomb that will otherwise be revealed when you strip the forms.
Then you need a float, which is a board with a handle to work the surface. You will "float" fines and water to the top, but not too much. If you start with a fairly stiff mix, you should be able to float fines and water to the top to make a smooth finish.
After it starts to set up a bit, you usually broom it so it isn't too slippery.
When it gets stiff but is still workable, you edge it and put crack grooves across it at about 4 ft intervals, and at the intersection of any inside corners. Inside corners always crack and you want to force the crack straight across the sidewalk.
After the concrete sets up, you want to keep it moist for at least a week. Too much water in the mix makes poor concrete, but drying after it sets up makes it weak. The cement in concrete (please don't call concrete "cement") forms a hydrate with water when it cures. No water in curing means no hydration which means poor concrete.